B Why is the age of the Universe finite?

I am assuming scientists say that the universe is 13.8 billion years old with respect to time on earth. If so, how come the infinite energy present at such a small scale didn't make gravitational time dilation infinite during the first stage? Wouldn't time dilation make the universe infinitely old with respect to earth?

If so, how come the infinite energy present at such a small scale didn't make gravitational time dilation infinite during the first stage?

i think you are referring to Big- bang , naturally energy was large say infinite but that would make time interval 'smaller' if uncertainty remains or holds.
the finite age gets calculated from the Big bang.....and CMB radiation does point to it/gives an estimate of finite age.

Staff: Mentor

i think you are referring to Big- bang , naturally energy was large say infinite

The energy density of the universe at the time of the Big Bang (which is not, btw, the "initial singularity"--there was no such thing--but the hot, dense, rapidly expanding state at the end of the inflation epoch, which is the furthest back that we have reliable evidence) was much larger than it is now, but still finite. However, that does not mean the "energy" as a whole was much larger. There is no well-defined notion of "the energy of the universe". Only its energy density is meaningful.

that would make time interval 'smaller' if uncertainty remains or holds.

If you are referring to the energy-time version of the quantum mechanical uncertainty principle, first of all, that's not as simple as it looks, and second, it is irrelevant to the concept of time dilation in relativity.

Wouldn't time dilation make the universe infinitely old with respect to earth?

I think time dilation describes the difference between two clocks. If you can describe the two clocks you are picturing that prompted you to ask this question, you can probably get a better explanation of why time dilation does not make sense in this context.

There can be no physical meaning to a clock that is outside the universe, for instance.

I wonder if all clocks in the universe tend to agree more and more as the age of the universe (wound-backwards) is approaching time zero - that is something I had never thought about before.

Staff: Mentor

More precisely, it describes the difference between two clocks given some way of telling which pairs of events on the two clocks' worldlines happen "at the same time". But in most cases there is no such way that is picked out by the physics; it's just an arbitrary convention (the usual term is "simultaneity convention"). In the case of cosmology, the only simultaneity that is picked out by the physics is the convention that is natural for "comoving" observers; and all comoving observers have zero time dilation relative to each other. So in the only case where "time dilation" has a well-defined meaning in cosmology, it is not present at all.

but the hot, dense, rapidly expanding state at the end of the inflation epoch, which is the furthest back that we have reliable evidence) was much larger than it is now, but still finite

Ok now I know why the time dilation wasn't infinite. But wouldn't the much higher energy density, and therefore much stronger gravitational field, mean that time was dilated as compared to Earth's time rate as it is now?

Staff: Mentor

wouldn't the much higher energy density, and therefore much stronger gravitational field, mean that time was dilated as compared to Earth's time rate as it is now?

No. As I said, the concept of "time dilation" does not apply in this context. Neither does the concept of "gravitational field" as you are using it here; that would require a stationary spacetime, and the universe as a whole is not a stationary spacetime.

The energy density of the universe at the time of the Big Bang (which is not, btw, the "initial singularity"--there was no such thing--but the hot, dense, rapidly expanding state at the end of the inflation epoch, which is the furthest back that we have reliable evidence) was much larger than it is now, but still finite. However, that does not mean the "energy" as a whole was much larger. There is no well-defined notion of "the energy of the universe". Only its energy density is meaningful.

If you are referring to the energy-time version of the quantum mechanical uncertainty principle, first of all, that's not as simple as it looks, and second, it is irrelevant to the concept of time dilation in relativity.

The universe does have a finite age, but it's not the time since the CMB was emitted; that happened about 380,000 years after the Big Bang.

There seems to be a contradiction here, you say that there is much such thing as the singularity. but you also say the universe has a finite age. But how do you get to the finite age without assuming there was a singularity? I presume you mean there is a finite amount of time in the past we can reliably probe, i.e. before 13.8 billion years we don't know what was going on, after that we know what was going on, before we don't, is that right? i don't see this is the same as saying the universe has a finite age. i think that is totally incorrect.
i think the correct statement should be the age of the universe is unknown. we can probe back 13.8 years, beyond that the universe may have continued to exist but we don't know.

Staff: Mentor

how do you get to the finite age without assuming there was a singularity?

The "age" we are talking about is the time since the Big Bang (please re-read the definition I gave of that term, it's important); that is finite. The Big Bang is not an initial singularity. We don't know exactly what came before the Big Bang; it's possible that there was an infinite time before it. It is also possible that the concept of "time" doesn't have any meaning before it (or more precisely before the inflation epoch, which ended with the Big Bang). When we say the universe has a finite "age", we are not making any claims about all that; all we are saying is that the Big Bang was a finite time ago.

This conversation reminds me a bit of a thought that ran through my head a couple of weeks ago while listening to a podcast with Katie Mack.
She was trying to explain something similar to a 9 year old, which is about my level of understanding of the universe, so I thought I might learn something.
Absolutely nothing made sense. TOO many infinities.....

But anyways, her conversation reminded me of that old image of em-waves,

and how the big band/big crunch thing, kind of looked very similar, as everything appears to disappear into nothingness, every half-wave.

I am assuming scientists say that the universe is 13.8 billion years old with respect to time on earth. If so, how come the infinite energy present at such a small scale didn't make gravitational time dilation infinite during the first stage? Wouldn't time dilation make the universe infinitely old with respect to earth?

Because the age is measured with respect to a particular hot, dense state that occurred 13.8 billion years ago. No reference is made to what may or may not have happened before that time (anything that did happen is speculation, as information about what happened earlier, if there was an earlier, was destroyed by the nature of this early hot, dense state).

The "age" we are talking about is the time since the Big Bang (please re-read the definition I gave of that term, it's important); that is finite. The Big Bang is not an initial singularity. We don't know exactly what came before the Big Bang; it's possible that there was an infinite time before it. It is also possible that the concept of "time" doesn't have any meaning before it (or more precisely before the inflation epoch, which ended with the Big Bang). When we say the universe has a finite "age", we are not making any claims about all that; all we are saying is that the Big Bang was a finite time ago.

This might be your opinion, but for better or worse, it is not the standard usage in cosmology. See above.

My problem is not saying there is 13.8 billion years since the big bang , we all agree on that. But thats a different statement to saying the universe is 13.8 billion years old. To say that implies that the universe did not exist before the big bang. I believe that is what many cosmologists believed due to the Penrose Hawking singularity theorems. Many cosmologists have told the public that the universe did not exist before the big bang; that asking what happened before the big bang is like asking what is north the north pole. By saying the universe is 13.8 billion years old (which is what you did say) rather than its 13.8 billion years since the big bang ( which you have indicated is what you meant) I believe you reinforce this incorrect view. There are several professional well respected cosmologists who have said other cosmologists are misleading the public , Sean Carroll for example:

"They (cosmologists) will go to great effort to explain how the Bang was the beginning of space and time, that there is no “before” or “outside,” and that the universe was (conceivably) infinitely big the very moment it came into existence, so that the pasts of distant points in our current universe are strictly non-overlapping. All of which, of course, is pure moonshine. When they choose to be more careful, these cosmologists might say “Of course we don’t know for sure, but…” Which is true, but it’s stronger than that: the truth is, we have no good reasons to believe that those statements are actually true, and some pretty good reasons to doubt them.""http://www.preposterousuniverse.com/blog/2007/04/27/how-did-the-universe-start/

To say that its standard in cosmology to talk about the universe being 13.8 billion years old is not a defence , its an admission of a problem in that way that cosmologists ( some not all) communicate to the public. It's an admission the problem of cosmologists misleading the public got much wider than just this discussion. As this web site presumably is about cosmologists communicating to the public it should be a good place to put right a bad practise and not re-inforce a misleading statements.

By saying the universe is 13.8 billion years old (which is what you did say) rather than its 13.8 billion years since the big bang ( which you have indicated is what you meant) I believe you reinforce this incorrect view.

Speaking for myself, I found the discussion of what it means to talk about the age of the universe helpful and thought provoking. I did not / do not feel misled.

I do feel that it is difficult to be simultaneously precise and lay-person comprehensible; I suggest we allow some breathing space to folks who add precision to their statements from one post to the next.

I am very glad that you, Grinkle did not feel misled. But it doesn't follow that the continual use of phrases like : "the universe is 13.8 billion years old" isn't continuing to mislead the public. Some like Sean Carroll or the Max Planck Institute here:http://www.einstein-online.info/spotlights/big_bangs are trying to undo the bad work. Hopefully others will follow suit.

Staff: Mentor

@windy miller: Your issue is with terminology, not physics. We all agree on the physics. PF is not the place to argue about terminology; that's not what we're here for. If the mainstream usage changes, PF will change with it.